The present invention relates generally to gas turbine engines, and, more specifically, to combustors therein.
In a gas turbine engine, air is compressed in a compressor, mixed with fuel in a combustor to generate hot combustion gases which flow downstream through one or more turbine stages which extract energy therefrom. A high pressure turbine powers the compressor, and a low pressure turbine typically powers a fan disposed upstream of the compressor for producing thrust in a turbofan engine for powering an aircraft in flight.
The combustion of fuel and air produces exhaust emissions including carbon monoxide (CO), unburned hydrocarbons (HC), smoke, and nitrogen oxides (NOx). Government regulations have become increasingly more stringent in limiting undesirable exhaust emissions in commercial aircraft. As a result thereof, gas turbine combustors undergo continual development for reducing undesirable exhaust emissions while maintaining suitable performance of the combustor and an effective useful life thereof.
For example, a significant improvement in combustor design was the introduction of double dome combustors replacing single dome combustors. Both combustors include an annular radially outer combustor liner spaced from a radially inner combustor liner, which are joined to an annular dome at upstream ends thereof.
In the single dome combustor, a single row of air swirlers and cooperating fuel injectors is mounted around the circumference of the dome for providing a number of fuel and air injection sites for generating the combustion gases. The combustor liners are air cooled using various forms of film cooling, and also include one or more rows of dilution air holes through which additional air is injected into the combustion gases for dilution thereof and producing desired radial and circumferential temperature profiles and pattern factors at the outlet of the combustor.
The double dome combustor includes two radially spaced apart rows of air swirlers and fuel injectors in the dome, with a cooperating annular centerbody extending downstream from the dome between the two rows to define two local combustion zones, i.e., an outer pilot zone, and an inner main zone. The double dome combustor is substantially shorter in axial length than a comparable single dome combustor and includes substantially more fuel injection sites for greatly reducing undesirable exhaust emissions.
However, the double dome combustor is substantially more complex than the single dome combustor, includes more components, and is operated differently for achieving low exhaust emissions, in particular CO, HC, and NOx, with suitable efficiency in operating performance over a varying power range from low-power idle to high-power takeoff in an aircraft engine application. Since the swirlers are fixed flow-area devices, they may be sized optimally at only one design point over the entire operating range of the combustor.
Air flow through the swirlers increases with increasing flow rate of the compressor air over the increasing power settings of the engine. And, the fuel to the corresponding fuel injectors may also be varied over the operating range of the combustor for varying the resulting fuel to air ratio for acceptable performance. A stoichiometric fuel to air ratio is a theoretical ratio for complete combustion, with lower ratios being considered lean, and higher ratios being considered rich. Rich combustion improves engine operability, whereas lean combustion reduces exhaust emissions and is limited to prevent lean blowout of the combustion gases.
Accordingly, in order to effectively operate a lean double dome combustor over its entire operating range, fuel staging between the pilot and main zones is required. Correspondingly, the outer fuel injectors of the pilot zone are separately joined to a common distributing fuel manifold for providing fuel thereto. And, the radially inner fuel injectors of the main zone are joined to one or more independent fuel manifolds for providing fuel thereto. In a typical application, a complex and expensive main staging valve controls fuel flow to the pilot and main fuel injectors. The main injectors may be arranged in two groups each fed by a common fuel manifold requiring a second staging valve to control fuel flow thereto.
In operation, only the pilot fuel injectors are provided with fuel at idle for enhancing ignition capability and reducing exhaust emissions at idle, with the main fuel injectors being provided with fuel above idle in stages up to full power operation of the combustor. The pilot and main swirlers are typically sized for achieving substantially complete combustion with the respective portions of the fuel injected therein, with the one or more rows of dilution holes providing quenching of the combustion gases to control the discharge temperature profiles thereof.
However, at idle, the main fuel injectors are off while air still flows through the corresponding inner swirlers into the combustor. An axially elongate centerbody is therefore required between the pilot and main swirlers and is attached to the dome for separating the two local pilot and main combustion zones for permitting effective operation of the combustor.
In view of the different operating requirements of the pilot and main fuel injectors and swirlers, the main injectors and swirlers are considerably larger in size and flow rate capability than the pilot injectors and swirlers in order to provide the considerable flow rates required for high power operation of the combustor. This, too, also increases the complexity of the combustor design and its operation.
In view of the complexity of the staged double dome combustor, it is desired to decrease the complexity thereof while achieving further reductions in exhaust emissions.
A gas turbine engine combustor includes outer and inner liners joined together at an annular dome. Outer and inner air swirlers and cooperating fuel injectors are mounted in two rows in the combustor dome. The fuel injectors are joined to a common fuel manifold for simultaneously channeling fuel thereto over an operating range from idle to full power for reducing exhaust emissions.